The present invention relates to a sputtering target for depositing silicon layers in their nitride or oxide form by means of reactive cathode atomization, such as e.g. Si3N4 or SiO2 in the form of optical functional layers on glass substrates. More particularly, according to the present invention, the target material is a cast silicon element, that has been solidified from the melt condition, with an additive mixed in with the melt. In another aspect, the present invention relates to a process for preparation of the aforesaid sputtering target and products obtained thereby
Thin silicon layers in their nitride and oxide form are becoming increasingly important in modern glass coating processes. In particular, the technology is important in the fields of optical coatings on large glass surfaces, as well as architectural, automotive and anti-reflective glass. These layers are usually applied by means of sputtering technology. A Si3N4-like or SiO2-like layer is deposited from a lightly doped silicon target via the extra addition of N2, as the reactive gas. For flat sputtering cathodes, use is usually made of highly pure, polycrystalline, boron-doped or phosphorus-doped silicon as the target material.
A process is known for the application of optically transparent layers, which comprise compounds of silicon on planar substrates, by means of reactive cathode atomization of silicon-containing target materials (EP 0,165,413). In that process a cast polycrystalline silicon molded element, which solidified from the melted condition, and which comprises at least 99% silicon together with dopants selected from the boron, antimony, phosphorus and arsenic group that are mixed into the melt, is atomized in an atmosphere which contains a reactive gas selected from the group oxygen and nitrogen, by means of a direct current.
With prior technology, the boron or phosphorus doped polycrystalline silicon tiles have been found to be rather brittle and expensive. For example, such targets can only be used as bonded targets. Moreover, such targets are in short supply because they are predominantly produced for photovoltaic wafers, instead of sputtering targets.
It is also known that plasma-sprayed silicon targets can be used for rotating cathodes on tubular steel supports. U.S. Pat. No. 5,853,816 shows that aluminum can be added to silicon sputter targets but points out that with aluminum insufficient homogeneity cause arcing of the SiAl target. The reference proposes to produce silicon particles coated with a conductive material such as Al, Ni or an Alxe2x80x94Ni alloy and then vacuum plasma spraying these particles onto the sputtering target carrier.
The highly pure, silicon target material, that is used for flat cathodes, is generally obtained as a by-product from the preparation of silicon slices for photovoltaic technology. For this purpose, slowly crystallized tiles are reworked for equipping the cathode. These tiles are then joined together by soldering or adhesion techniques to give a large cathode. The tiles produced in this way are more expensive in production, and they are brittle and therefore easily cracked by sputtering or in handling. A considerable problem occurs because these prior known titles need to be bonded, i.e., soldered, to a metallic backing plate for fixation in the sputter machine. The reason that such tiles are expensive is because they are produced by a very time consuming melting process and by an expensive cutting and grinding process.
A sputtering target is also known, especially with large dimensions and comprising brittle materials such as e.g. Si, Ge, In2O3 and SnO2 (DE-OS 19,527,476), which consists of a target base plate and a layer, which is arranged between the sputtering material and the base plate. The layer comprises an adhesive, preferably an epoxy resin adhesive, which has been mixed with a metal powder, whereby the target material is applied directly to the base plate with the intermediate inclusion of the adhesive layer, and the metal powder is formed from copper, nickel, gold or iron.
Finally, a target is known for the cathode of a device for coating work pieces (DE-OS 4,242,079), where the cathode is arranged in a processing chamber, which is capable of being evacuated and filled with a processing gas, together with a cathode element and with a number of target tiles. These are contiguously arranged in a row on the cathode element and which collectively form the target. There is at least one target base plate, which is arranged between the target tiles and the cathode element, whereby the target tiles, that are formed from the material that is to be sputtered off, each have a plate like or parallelepiped configuration with plane parallel faces and are firmly soldered, welded or glued in each case to a target base plate with similar dimensions. The base surfaces of the target base plates are designed to be slightly larger in their length than the base surface of the target tiles that are associated therewith in each case, whereby the two edge parts across the breadth of the target base plate, which project beyond the edge of the target tiles in each case, are held firmly on the cathode element with the help of grasping clips.
An object of the present invention is to create a target which is suitable for the preparation of silicon layers in their oxide or nitride form on a glass surface and which, in particular, is especially inexpensively capable of being molded into the configuration of tiles.
It is a further object of the present invention to avoid the need for bonding silicon sputtering targets to a base plate.
The above and other objects of the invention can be achieved by a target material which comprises silicon with the additive concentration of 1 wt % to 15 wt % aluminum. A molten melt of the silicon and aluminum is prepared. This material is then cast into a flat mold, which can take the shape of a tile on the solidified melt, preferably the configuration of a flat planar molded element with parallel faces.
The basic target material, to which the concentration of the aluminum has been mixed in with the silicon, is cast in a flat casting mold which is close to the final size of the desired target. The solidified semi-finished products, which have been cast closely to their final size, can be reworked into individual target tiles at comparatively low expense in terms of a minimum of grinding or cutting and are then assembled to provide the desired configuration.
Surprisingly, it has been found with sputtering targets comprising these cast silicon-aluminum tiles, which are close to the final size, that silicon oxide or silicon nitride layers can be sputtered with comparable optical parameters despite the casting defects, that are present, such as micro-pores and micro-fissures and despite the high aluminum doping level.